There is a simple time saver for sketching to quickly transition from a line to an arc.
When sketching a line you can transition from a line to an arc and back again using one of the following methods:
1) After you have sketched a line, but while the line tool is still active, you can press “A” key on your keyboard. This which will transition to the line tool to the arc sketch entity. Pressing “A” on the keyboard again will return you back to the line tool.
2) While sketching your line, you can bring back your cursor to hover on the endpoint of the prior line. This auto-transitions to the arc tool. Returning your cursor to hover on the endpoint of the prior line and clicking your mouse button will transition you back to the line tool.